Induction logging of earth formations from a borehole is an established commercial procedure. In such operations, a constant intensity, magnetic field was generated for propagation into the adjacent formation, by continuously driving a source antenna with an alternating current at a constant frequency and a low steady state power level. A receiver coil assembly was usually electrically balanced to respond to induced eddy currents in the adjacent earth formation. The secondary magnetic field resulting from the eddy currents was then used to generate a voltage signal in the receiver coil assembly. The detected voltage signal varied in accordance with the conductivity of the adjacent formation. Usually, only the component of detected signal voltage that was in-phase with drive current was employed to indicate the formation conductivity or resistivity as an amplitude vs. depth trace.
Various technical papers and publications have discussed the operating principles of induction logging systems as described above. If the proper precautions were undertaken, the continuously detected voltage signal was directly and linearly proportional to the electrical conductivity or resistivity of the logged formation normalized to the range of formation values usually encountered.
Certain operational effects tended to adversely affect the accuracy of the data provided by such prior logging systems, however. One such non-linear effect resulted primarily from mutual interaction of different portions of the eddy currents, a so-called "skin effect", which varied as a function of frequency of operation, the effective length of the source and receiver coil array, and the conductivity of the adjacent formation. Although the occurrence of these objectionable variations could be substantially reduced or eliminated by proper choice of operating frequency and effective coil system length, such restraints limited desirable objectives of the system. For example, to increase the range of the logging system in a lateral direction, the effective system coil length must also be increased. A larger coil spacing, viz., between the source and receiver array, increased the non-linearity of the resulting detected signal due to skin effects.
There have been several methods proposed to correct for the aforementioned skin effect problems. In one such system, a function circuit was used to correct the detected signals in accordance with a predetermined function. In another system, the phase-quadrature signal (said to be approximately equal to the skin effect component of the in-phase detection signal over a given range of conductivity and frequency values), was eliminated.
Another adverse effect also limited the accuracy of results of conventional induction logging systems. This effect related to the fact that (i) the adjacent formation can be heterogeneous, (i.e. a plurality of conducting zones may exist in the adjacent formation other than the true formation conductivity) and (ii) the strata above and below the formation of interest, may effect signal response. Such conditions created substantial errors in the accumulated data using prior art techniques.
Yet another adverse effect arose when the borehole was filled with a drilling mud which formed mud cake along the sidewall of the borehole and permitted a filtrate to invade the formation and form a so-called invaded zone around the borehole. As a result the diameter of the borehole also varied so that the logging sonde was offset different lateral distances from the mud cake as data was taken, introducing yet another adverse data effect.
To overcome the aforementioned adverse effects in part, different arrays and associated circuitry were designed in prior art systems to provide a plurality of different radial logging devices. The separate devices, however, had to be designed so that their operations were independent in attempts to compensate for various adverse effects. For example, individual signals were often compensated for either by tornado charts or by time domain computational methods, such as by the addition of weighting factors. These stored weighting factors combined in such a way that the effect of regions other than the region under consideration, were diminished.
While prior art alternating current induction logging systems were effective, their utility was greatly restricted by their limited lateral range, their limited vertical resolution, their limited accuracy in determining true formation conductivity, and their inability to determine dip angle of beds or the range and azimuth of formation anomalies.
Accordingly it is the principal object of this invention to provide a new and improved induction logging method and apparatus whereby logs of greatly improved accuracy, range, resolution, and reliability may be obtained.